As one background example of an application or operating environment, highly sensitive, and valuable, telecommunications equipment, such as that used in the operation of mobile telephone base stations and radio towers is often housed in outdoor housings. These housings may vary significantly in size and construction. Some are built on the ground, equipped with doors, and are large enough to shelter the repair and maintenance personnel who service the equipment. Other housings are smaller and may be located off of the ground and part of the way up and mounted directly to a radio tower such as metal casing mounted to the radio tower. Therefore, there are different ways to provide for the telecommunication base station with different types of telecommunication electronics housings. These telecommunication base stations are often located all over the world, many in quite remote locations.
Because the electronic equipment housed in these housings typically use high levels of electrical current during operation, a significant amount of heat may be generated within the housing. To ensure that the continued operation of this highly sensitive and valuable equipment is not adversely affected by excessive heat buildup, the housing may include some type of cooling system to keep the temperature in the housing within an acceptable range. Such a cooling system may include one or more fans designed to pull in cool air from the atmosphere outside of the housing, while exhausting the warm air inside the housing. Alternatively, some housings may simply provide a flow path for cool air from the outside through the housing to cool the electronic equipment housed therein.
Due to the aforementioned sensitivity of the electronic equipment housed in these housings, it is important or highly desirable that the housings prevent outside elements, such as water, dirt, and dust that might harm the equipment from entering the housing. In some systems, membrane filters are used to ensure the equipment is adequately protected from water, dirt, and dust particles. These systems are generally sealed housings, but for an opening for the membrane filter and outlet port for exhausting internal air. Other systems are completely sealed and known as heat exchanger systems.
It should be recognized that these systems may operate in a wide range of different climates and environments depending upon where a given telecommunication base station and radio tower is located. For example, many of these telecommunication base stations are provided near an ocean and therefore can be subject to mist or fog containing salt, sometime referred to as “salt fog”. Accordingly, operators of such telecommunication base stations will typically have certain requirements for the protection of their electronic equipment contained within the base stations. In fact, one typical test employed is known as the “salt fog test”, in which the entire cabinet (in other words, the housing with the filter mounted therein) shall be placed in an environmental test chamber and exposed to a salt fog spray for 30 days, consistent with GR-487-CORE and in accordance with ASTM B 117. During this test, the fans are operated to draw air flow including the salt fog into the filter element. Given the small size of moisture particles in salt fog and fluid nature of water (water being in liquid form is readily deformable and can expand or contract in size easily; unlike traditional dust particles, which are solids and are considered to have a fixed size), and the complications associated with salt, it can be seen that it would be quite difficult to prevent the passage of moisture through the filter media with a fan drawing air into the housing. This is especially so when it is desirable to move air into the housing free of mist/fog or other water, dust and particles so as to cool the inside of the housing. Therefore, an overly restrictive filter would not be desirable. Yet further, temperature variations and temperature fluctuations can further create difficulties.
While one background example is discussed in some detail above, it should be recognized that beyond the telecommunications industry, there are numerous different applications for where it is desired to vent a housing enclosure with outside air that may carry dust, moisture and/or salt, without carrying such dust, moisture and/or salt into the housing enclosure. For example, manufacturing facilities, clean rooms, material storage warehouses, and numerous other electronic housings also are vented with filtered external air. Accordingly, various different venting arrangements and environments are known.
The prior art attempts to providing membrane filter elements for telecommunication base stations and other such applications are disclosed in U.S. Pat. No. 6,885,554 to Reeck et al. (assigned in part to W.L. Gore & Associates GMBH) and EP 1,750,493 A1 to W.L. Gore & Associates GMBH (herein “Gore”), both of which are incorporated by reference to the extent not inconsistent with the present disclosure. Other earlier patents also disclose membrane filter elements for such water tight housing enclosures housing electronics and the like, such as U.S. Pat. No. 6,885,554 to Kobayashi; JP Publication No. 04-0338794 to Kenji; JP H06-031130 to Tsutsumi; U.S. Pat. No. 5,507,847 to George; U.S. Pat. No. 5,901,034 to Fuglister; and EP 0,395,331 to Ichiyasu.
However, usually such prior attempts disclose only and are restricted to using “membrane” type filters, which according to earlier patent records of Gore (e.g. U.S. Pat. No. 6,218,000), that apparently comprise a process of extruding a thin rectangular tape film of PTFE (aka “Teflon®) from a pellet and then stretching the film to create pore structures within the film. While commercially available membrane type Filters® from Gore are currently believed to pass the salt fog test standard, these filters like the patents require a special membrane material, which are restrictive due to the tight film structure and load very quickly due to the membrane's surface loading nature, thereby reducing lifespan and reducing airflow quickly over time due to increased restriction. Different environments will experience different types of dust particles including different gradient densities of different particle sizes. As such, these types of filters may not be optimal for many different climates and operational environments that may exist across the globe. There also is some discussion of non-membrane materials with water repellency such as in U.S. Pat. No. 6,885,554 to Kobayashi, which mentions porous membranes or Teflon, but alternatively water repellant fiber materials of various sorts.
Other prior art attempts for cooling watertight enclosures as mentioned in the '554 patent are sealed heat exchanger systems that utilize a heat exchanger to cool inside air as opposed to inletting fresh outside air into the electronics housing. These types of systems however do not employ a filter, as fresh air is not allowed to penetrate the housing. Instead, separate exterior and interior fans circulate air through the heat exchanger. These systems are thus typically more expensive and further are subject to the efficiency of the heat exchanged in the heat exchanger unit.
Various industry test standards have developed for telecommunications housings or other enclosures incorporating cooling systems, that are applied to both heat exchanged units as well as membrane type filtration units. These may include for example a wind driven rain intrusion test, a rain intrusion test (no wind, but heavy rainfall), a lawn sprinkler test, a weather tightness and dust intrusion test, wind resistance test, impact resistance test, fire resistance test (e.g. due to brush fires), and a corrosion resistance or salt fog test. It is the salt fog test that often proves to be most difficult as a filter may need to pass 30 days of being subject to salt fog. For example according to one test, the entire cabinet shall be placed in an environmental test chamber and exposed to a salt fog spray for 30 days, consistent with GR-487-CORE and in accordance with ASTM B 117. Fans, used to circulate outside air (i.e., heat exchanger fans), are in operation during this exposure. Only a filter that prevent moisture (carrying salt) from breaking through the filter element and into the cabinet survives this rigorous test. Other tests for filters that present serious difficulties are the simulated rain tests.
The present invention is directed toward improvements over the state of the art.